Remove Observables and switch to Channels

Nobody has claimed this yet.

Our current implementation for scanning files/directories is done by using observables:

https://github.com/microsoft/component-detection/blob/29e89522bb3c243be4aa8e188ce2b8c57e96420b/src/Microsoft.ComponentDetection.Common/FastDirectoryWalkerFactory.cs#L32

Instead, we should architect a channel system that works as:

1. Create a new Task & Channel for each detector
2. Launch each detector with a semphore blocking execution untill all detectors have subscribed to the channel
3. Begin scanning directories
4. Each file found will get sent through the channel
5. Detector gets the message, and scans file or ignores it depending on the pattern

We could also implement a Broadcast channel similar to https://docs.rs/tokio/latest/tokio/sync/index.html#broadcast-channel

AB#2102424

Start with src/Microsoft.ComponentDetection.Common/FastDirectoryWalkerFactory.cs, especially the observable-based scanning implementation linked in the issue. Trace how detectors subscribe and consume files, then define the channel lifecycle and synchronization; the work is done when directory results reach each detector through channels without the existing observables.

Written by the indexing model from the issue text.